1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount suitably used for example as an engine mount for flexibly mounting on a motor vehicle a power unit which includes an internal combustion engine. More particularly, the present invention is concerned with a fluid-filled cylindrical engine mount which exhibits desired spring stiffness in the axial direction, without deterioration of damping/isolating characteristics with respect to input vibrations applied in the radial or diametric direction, and which is constructed for easy tuning of the diametric and axial spring characteristics relative to each other.
2. Discussion of the Prior Art
A so-called cylindrical elastic mount is known as one type of a mounting device which is interposed between two members of a vibration system for flexibly connecting the two members in a vibration-damping or -isolating fashion. This cylindrical elastic mount is constructed with an elastic or rubber body being formed between an inner metal sleeve, and an outer metal sleeve disposed radially outwardly of the inner sleeve with a suitable radial spacing therebetween. The elastic body elastically connects the inner and outer sleeves. The thus constructed cylindrical elastic mount is widely used as an engine mount for a motor vehicle, since the mount has various advantages, such as being compact and simple in construction and having a reduced size, and comparatively limited relative displacement of the inner and outer sleeves upon application of a vibrational load having a large amplitude.
Recently, there has been proposed a so-called fluid-filled cylindrical elastic mount as disclosed in U.S. Pat. No. 4,749,173 to R. Kanda owned by the assignee of the present application. Such a fluid-filled cylindrical elastic mount has a pressure-receiving chamber and an equilibrium chamber, which are formed between the inner and outer sleeves and are filled with a suitable non-compressible fluid. The equilibrium chamber is at least partially defined by a flexible diaphragm, so that the volume of the equilibrium chamber is variable. The two chambers are held in communication with each other through an orifice passage. Upon application of a dynamic vibrational load between the inner and outer sleeves of the elastic mount, the fluid pressure within the pressure-receiving chamber periodically changes, whereby the fluid is forced to flow through the orifice passage between the two chambers. Based on the resonance of the fluid mass flowing through the orifice passage, the fluid-filled elastic mount provides an excellent vibration damping/isolating effect, which cannot be obtained from a cylindrical elastic mount which relies on only the elasticity of an elastic body to damp the input vibrations.
In the fluid-filled cylindrical elastic mount as described above, the deformation of the elastic body occurs primarily as a compressive strain when the mount receives a vibrational load applied in the radial or diametric direction, and primarily as a shearing strain when the mount receives the vibrational load applied in the axial direction. This means considerably reduced spring stiffness or considerably soft spring characteristic in the axial direction, as compared with the spring stiffness in the diametric direction. Usually, the ratio of the spring stiffness in the axial direction to that in the diametric direction falls within a range of about 0.2-0.6, and cannot be increased beyond the upper limit of that range.
If the elastic body is formed of a rubber material having increased hardness in an attempt to increase the spring stiffness in the axial direction, the spring stiffness in the diametric direction tends to be excessively high, causing an accordingly reduced amount of compressive deformation of the elastic body upon application of the vibrations to the mount. This results in reduction in the amount of volumetric change of the pressure-receiving chamber and an accordingly reduced amount of change in the fluid pressure in the pressure-receiving chamber, leading to a decrease in the fluid mass which is forced to flow through the orifice passage. Thus, the solution indicated above leads to insufficient vibration damping/isolating capability of the elastic mount.
In other words, the known fluid-filled cylindrical elastic mount inevitably suffers from undesirably low spring stiffness with respect to the vibrations applied in the axial direction, if the elastic mount is adapted to exhibit sufficiently high vibration damping/isolating capability with respect to the vibrations applied in the diametric direction. Consequently, if this known cylindrical elastic mount is used as an engine mount for an automotive vehicle, the power unit of the vehicle tends to be oscillated relative to the vehicle body in the axial direction of the elastic mount, deteriorating the driving comfort of the vehicle. The known fluid-filled cylindrical elastic mount has the problem discussed above.